1. Field of the Invention
The present invention relates to a device for limiting slip of a torque converter for an automatic transmission in a high temperature condition.
2. Description of Related Art
A slip limiter device conventionally used as countermeasures against high temperature is disclosed, for example, in JP 04-151069A, which is applied to a lockup-type torque converter for an automatic transmission. As generally known in the art, a lockup-type torque converter is changed from a converter mode to a lockup mode in which an input element and an output element are directly coupled to each other (i.e., the slip rotation of the torque converter is zero) in a vehicle speed zone above a predetermined lock-up vehicle speed, where torque multiplying function and torque fluctuation absorbing function of torque converter are not required. The slip limiter device as disclosed in JP 04-151069A is operative when the hydraulic oil is in a high temperature condition, to lower the lock-up vehicle speed so that the torque converter can be readily locked-up thereby suppressing heat generation due to intense agitation of the hydraulic oil by the torque converter.
On the other hand, if the countermeasures against high temperature and overheating as described above are implemented based on the hydraulic oil temperature, these countermeasures are proper to an extent that a high-load operation of the vehicle is continued for a long period of time. In other words, when the high-load operation of the vehicle takes place only temporarily, even though a high temperature has been once attained, the temperature decreases in a short time so that the countermeasures tend to cause undesirable hunting. From such a viewpoint, an improvement has been proposed in JP 07-229556A, for example, wherein the high temperature countermeasures are implemented only when the high-load operation of the vehicle is continued for a long period of time.
However, if the torque converter is locked up at the time of high-load operation in this manner, the slip rotation of the torque converter becomes zero and, hence, the torque fluctuation absorbing function is not achieved at all, so that booming noise and/or shaking vibration that are liable to occur at the time of high-load operation are directly transmitted to a vehicle body, thereby deteriorating drivability or driving comfort.
On the other hand, if the torque converter is placed in a converter mode in which the slip rotation is not limited at all during a high-load condition, to thereby maintain satisfactory drivability or driving comfort, the hydraulic oil is intensively agitated by the torque converter so that the overheating countermeasures cannot be fully achieved.
It is therefore a primary object of the present invention to provide an improved slip limiter device, which effectively eliminates the above-mentioned problems of the prior art.
It is a more specific object of the present invention to provide an improved slip limiter device, by which the measures against overheating can be implemented without deteriorating the drivability, and thus both of the measures against deteriorated drivability and driving feel, as well as the measures against overheating can be taken at the same time.
In search for a solution of these objects, the inventors conducted thorough research and investigations and arrived at an insight that it would be highly advantageous to limit the slip of the torque converter in a high temperature condition, by expanding the slip control region in which the slip rotation of torque converter is decreased to a high-load operation region, instead of the conventional approach of shifting the lockup vehicle speed toward the low vehicle speed side and thereby expanding the lockup region in which the slip rotation of torque converter is made zero, such that agitation of the hydraulic oil is alleviated by the decreased slip rotation at the time of high-load operation to thereby implement measures against overheating without deteriorating the drivability and driving feel.
According to the present invention, there is provided a slip limiter device for limiting slip of a torque converter in a high temperature condition thereof, by carrying out slip control for reducing slip rotation between an input element and an output element of said torque converter in a low-load operation region of an engine, or a lockup control for directly coupling said input and output elements to eliminate said slip rotation, wherein said slip control is carried out in a region which is expanded to a high-load operation region of the engine when torque converter hydraulic oil is at a temperature not lower than a predetermined temperature.
With the slip limiter device according to the present invention, the slip control for reducing the slip rotation between the input and output elements of the torque converter in the low-load operation region of an engine is carried out, or the lockup control for eliminating the slip rotation is carried out. When the torque converter hydraulic oil temperature is not lower than the predetermined temperature, the region of the slip control is expanded to the high-load operation region.
Owing to the expansion of the slip control region to the high-load operation region, the torque converter has a tendency of decreased slip rotation during the high-load operation, so that agitation of the hydraulic oil caused by the torque converter is alleviated by the decreased slip rotation, thereby allowing the high temperature measures to be taken. Moreover, since such a functional effect can be achieved by the expansion of the slip control region, it is possible to avoid undesirable deterioration in drivability and/or driving feel, which had been inevitable when the lockup region is expanded in a conventional manner. Thus, both of the measures against deteriorated drivability and/or driving feel, as well as the measures against overheating can be taken at the same time.
It is preferred that the slip control region be expanded when the torque converter hydraulic oil temperature is not lower than the predetermined temperature at least for a predetermined time. By this, the expansion of the slip control region toward the high load side is performed only when it is really necessary, and not wastefully.
It is additionally or alternatively preferred that the torque converter is placed in a lockup mode in the expanded region when the torque converter hydraulic oil temperature is not sufficiently decreased even by the expansion of said slip control region. By this, when a decrease in the oil temperature due to the expansion of the slip control region is not found, the agitation of the hydraulic oil is further alleviated by the further decreased slip rotation, thereby allowing reliable overheat countermeasures to be taken.
In this instance, the control is preferably returned from the lockup control in which the torque converter is placed in the lockup mode, to the slip limiting control based on a normal slip limiting region pattern, when the torque converter hydraulic oil temperature is sufficiently decreased by the lockup control in which the torque converter is placed in the lockup mode in the expanded region. By this, the control for placing the torque converter in the lockup mode in the expanded region can be finished properly, and the restoration to the normal slip limitation can be effected in a reliable manner.
The restoration to the normal slip limiting control is preferably executed when the operating condition is in a region other than the expanded slip limiting region, so that the torque converter is in the same slip limiting mode upon restoration to the normal slip limitation and before such restoration, and the restoration to the normal slip limitation can be effected substantially without shock.
It is additionally or alternatively preferred that the torque converter is placed in the lockup mode in the expanded region when the expansion of the slip control region continues at least for a predetermined time. This serves to avoid a problem that the slip control mode is continued for an excessively long period of time by the expansion of the slip control region toward the high load side.
In this instance also, the control is preferably returned from the lockup control to the slip limiting control based on a normal slip limiting region pattern, when the torque converter hydraulic oil temperature is sufficiently decreased by the lockup control in which the torque converter is placed in the lockup mode in the expanded region, and the restoration to the normal slip limiting control is preferably executed when the operating condition is in a region other than the expanded slip limiting region, to thereby bring about the functional advantages mentioned above.
It is additionally or alternatively preferred that the control is returned from a slip limiting control based on an expanded slip limiting region pattern in which the slip control region is expanded, to a normal slip limiting control based on a normal slip limiting region pattern, when the torque converter hydraulic oil temperature is sufficiently decreased the expansion of the slip control region. By this, the control for making the torque converter in the lockup mode in the expanded region can be finished properly, and the restoration to the normal slip limitation can be effected in a reliable manner.
In this instance also, the restoration to the normal slip limiting control is preferably executed when the operating condition is in a region other than said expanded slip limiting region, to thereby bring about the functional advantage mentioned above.